Speakers vs Tubes - when do they play nice with each other

It's not as simple as tubes like high impedance speakers and vice versa.

Read this article, ... go catch some sleep and then read it again.
Wait a week.
Repeat
....

http://www.symphonysound.com/articles/tubefriendly.html

BlogWorthy - Crossover Mashup

Description of various crossover types and their respective advantages / disadvantages/
http://www.audioholics.com/education/loudspeaker-basics/filter-crossover-types-for-loudspeakers

In the same vein, here's a simple walk-through of designing an actual crossover for a 3 way loudspeaker
http://burningass.110mb.com/crossover.php

And lastly, a basic crossover calculator.
http://www.ajdesigner.com/crossover/crossoverfirst.php